Theheart is a muscular organ responsible for pumping blood through the
blood vessels by repeated, rhythmic contractions, or a similar structure in the
annelids, mollusks, and arthropods. The termca rd ia c (as in cardiology)
means "related to the heart" and comes from the Greek \u03ba\u03b1\u03c1\u03b4\u03af\u03b1,k a rd i a, for
"heart." The heart is composed of cardiac muscle, an involuntary muscle
tissue which is found only within this organ. The average human heart
beating at 72 BPM, will beat approximately 2.5 billion times during a
lifetime spanning 66 years.

The function of the right side of the heart (see right heart) is to collect de-
oxygenated blood, in the right atrium, from the body and pump it, via the
right ventricle, into the lungs (pulmonary circulation) so that carbon dioxide
can be dropped off and oxygen picked up (gas exchange). This happens
through the passive process of diffusion. The left side (see left heart) collects
oxygenated blood from the lungs into the left atrium. From the left atrium
the blood moves to the left ventricle which pumps it out to the body. On both
sides, the lower ventricles are thicker and stronger than the upper atria. The
muscle wall surrounding the left ventricle is thicker than the wall
surrounding the right ventricle due to the higher force needed to pump the
blood through the systemic circulation.
Starting in the right atrium, the blood flows through the tricuspid valve to
the right ventricle. Here it is pumped out the pulmonary semilunar valve and
travels through the pulmonary artery to the lungs. From there, blood flows
back through the pulmonary vein to the left atrium. It then travels through
the mitral valve to the left ventricle, from where it is pumped through the
aortic semilunar valve to the aorta. The aorta forks, and the blood is divided
between major arteries which supply the upper and lower body. The blood
travels in the arteries to the smaller arterioles, then finally to the tiny
capillaries which feed each cell. The (relatively) deoxygenated blood then
travels to the venules, which coalesce into veins, then to the inferior and
superior venae cavae and finally back to the right atrium where the process
began.
The heart is effectively a syncytium, a meshwork of cardiac muscle cells
interconnected by contiguous cytoplasmic bridges. This relates to electrical
stimulation of one cell spreading to neighboring cells.